Integrated circuits are often fabricated with multiple devices, such as transistors. To minimize integrated circuit size, and hence integrated circuit cost, these devices are often positioned in close proximity to one another. As a result, undesirable inter-device effects can arise. For example, undesired current can leak between devices. Alternatively, a device such as a transistor can switch on as a result of positive feedback between proximate devices. This effect is known as latch-up. Leakage current and latch-up are understood by persons skilled in the art.
To diminish these unwanted inter-device effects, it is desirable to adequately isolate proximate devices. Conventionally, inter-device isolation is accomplished by creating a field oxide between the devices. The field oxide is an electrical insulator. Thus, proximate devices are substantially electrically isolated if the field oxide has adequate dimensions including height, length, and width. However, if the field oxide dimensions are too small, leakage current and latch-up may result.
For example, the undesired inter-device effects may arise if a parasitic metal-oxide-semiconductor field effect transistor (MOSFET) is created between two adjacent devices. FIG. 1 illustrates one embodiment of a parasitic MOSFET 10 on an integrated circuit formed by a conductor 12, active areas 14, and a field oxide 16. The integrated circuit may be a memory, which contains memory cells and complementary MOSFETs. The active areas may be the source and the drain of memory cells or MOSFETs. The conductor 12 may be a wordline of the flash memory. The design and operation of flash memory are known by persons skilled in the art.
The structure of the parasitic MOSFET 10 is now described. The parasitic MOSFET 10 is unintentionally formed by elements of surrounding devices. The conductor 12 and the field oxide 16 function as a gate of the parasitic MOSFET 10. The active areas 14 serve as the source and drain of the parasitic MOSFET 10. Although it is not constructed like a conventional transistor, the parasitic MOSFET 10, nevertheless, may function like one if the field oxide 16 has sufficiently small dimensions. As a result, operation of the parasitic MOSFET 10 may cause undesirable leakage current and latch-up in surrounding devices. Therefore, it is necessary to maintain adequate field oxide 16 dimensions.
Methods of improving device isolation by enhancing field oxide 16 dimensions have been previously disclosed. U.S. Pat. Nos. 5,358,894 and 5,438,016 teach processes for reducing the thinning of the field oxide 16 thickness by respectively applying an impurity and using protective structures. However, these patents do not recite methods or structures that increase the depth that the field oxide 16 penetrates the substrate 18, while keeping the lateral encroachment to a minimum to maximize the area available for device fabrication. Many of these approaches also require multiple masking steps which increase processing costs. A process that minimizes the number of processing steps or masking steps is highly desirable.
In ULSI, devices will be positioned in closer proximity to one another than is done in very large scale integrated circuits (VLSI). However, with current technology, the field oxide 16 may be insufficiently deep, or in other words, does not penetrate sufficiently far into the substrate 18, to isolate the devices. Thus, undesirable leakage current and latch-up may occur in the devices. Therefore, a process and structure for isolating high density devices is necessary.
Furthermore, integrated circuits are fabricated with devices having microscopic, such as sub-micron, features that can only be manufactured with critical processing steps. The critical processing steps entail carefully aligning the substrate 18 to equipment used to build the devices. This requires that most processes leave the substrate 18 in a relatively planar configuration. Therefore, an integrated circuit fabrication process that is less sensitive to process variations is desirable. Such a process would permit successful fabrication of integrated circuits, despite minor misalignments.